Total knee arthroplasty involves replacement of portions of the patella, femur and tibia with artificial components. In particular, a proximal portion of the tibia and a distal portion of the femur are cut away (resected) and replaced with artificial components. In performing this knee surgery, it has been desirable to minimize the size of the incision to thereby minimize damage to soft tissue.
In particular, it is necessary to resect the proximal tibia and distal femur with, among other cuts, a facing pair of planer cuts from which other bone cuts, in the case of the femur, can be referenced.
In order to make these cuts, resecting the femur and tibia necessitates significant cutting of soft tissue, including muscles, tendons and ligaments.
As used herein, when referring to bones or other parts of the body, the term “proximal” means closer to the heart and the term “distal” means more distant from the heart. When referring to instruments, the term “proximal” means closer to the physician or user and the term “distal” means more distant from the physician or user.
There are several types of knee prosthesis known in the art. One type is sometimes referred to as a “resurfacing type.” In these prostheses, the articular surface of the distal femur and the proximal tibia are “resurfaced” with respective metal and plastic condylar-type articular bearing components.
One important aspect of these procedures is the correct resection of the distal femur and proximal tibia. These resections must provide planes which are correctly angled in order to properly accept and align the prosthetic components. In particular, the resection planes on the tibia and femur must be correctly located relative to at least three parameters: proximal-distal location; varus-valgus angle; and flexion-extension angle.
Moreover, following distal resection, the femur must be further shaped with the aid of a cutting block, the cutting block must be correctly located relative to internal-external rotation, medial-lateral position and anterior-posterior position.
Various computerized systems have been introduced to aid the practitioner during the surgical procedure by improving accuracy of the cuts. In one such system, the mechanical axis of the tibia and femur is identified with the system which then provides the ability to align the cutting block to the mechanical axis and after resection to verify the bone cuts. In these systems, multiple cameras are deployed above the surgical site to observe a plurality of dynamic reference frame devices also known as trackers. These trackers are attached to body parts and the surgical instruments and preferably include light emitting devices, such as light emitting diodes, which are visible to the cameras. Using software designed for a particular surgical procedure, a computer receiving input from the camera guides the placements of the surgical instruments with respect to the patient's mechanical axis. Such systems are shown in U.S. Pat. No. 6,385,475, the teachings of which are incorporated herein by reference.
Generally, the cutting block coupled to a tracker will be manually or free-handedly moved by the surgeon with the assistance of feedback from the computer system while the computer monitors movement of the tracker so that the block can be positioned in the desired position with regard to the patient's mechanical axis. However, finely adjusting the position of such devices even with the assistance of the computer system may still require substantial skill of the surgeon due to the manual or free hand nature of the movement or re-positioning of the block and tracker.
Improvements to such block and tracker devices can be desirable to maximize the potential of the positioning information of the computer system and to minimize the skill necessary to align the block and tracker in a minimally invasive soft tissue friendly incision so that the cutting guide will be located in the most advantageous position for a resection necessary during such an operation.